EM 1110-2-1100 (Part V)
31 Jul 2003
Conversely, short breakwaters at greater distance from shore favor salient formation. The
"Conditions for the Formation of Salients" in Table V-3-6 presents a wide range of conditions with
generally, Ls/Y < 1. Dally and Pope (1986) recommend for salient formation
Ls
' 0.5 to 0.67
(V-3-8)
Y
for both single and segmented breakwaters. However, for very long distances, to insure that tombolos do not
form, the recommended ratio for a segmented system (Dally and Pope 1986) is
Ls
(V-3-9)
' 0.125 (long systems)
Y
Permeable structure systems (partly submerged, large gaps) also allow sufficient wave energy to
minimize the chance for tombolo formation: As shown in Table V-3-6, "Conditions for Minimal
Shoreline Response," the references cited generally recommend Ls/Y< (0.125 0.33) to produce a
minimal shoreline response. Ahrens and Cox (1990) defined a beach response index, Is
Is = exp (1.72 - 0.41 Ls/Y)
(V-3-10)
where the five types of beach response (Pope and Dean 1986) give Is value as:
Permanent tombolo formation, Is=1
Periodic tombolos,
Is=2
Well-developed salients,
Is=3
Subdued salients,
Is=4
No sinuosity,
Is=5
These results are preliminary and require verification.
The ratio Ls /Lg is also important for salient or tombolo formation. Large gaps will let more wave
energy reach the shore to promote salient formation. And, this will coincide with smaller Ls /Lg
ratios. A dimensionless plot of U.S. segmented, nearshore breakwater projects by Pope and Dean
(1986) using Ls/Lg is shown in Figure V-3-22 and verifies this trend. The vertical axis is Y/ds and
is the distance offshore relative to the local, mean water depth at the breakwater. The water depth
is important for it is related to the nominal, surf zone width at breaking Yb. The dimensionless ratio
Y/Yb is a measure of breaker location relative to the width of the surf zone. For a given Ls /Lg, larger
Y/ds values (or Y/Yb) mean the breakwater is located further offshore (beyond the normal surf zone
width) to foster salient formation. Obviously, breakwaters located far offshore will have less
effection the shoreline.
Shore Protection Projects
V-3-53
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